The use of a monitor photosensor, such as a photodiode, positioned to receive light emitted from the rear facet of an injection laser is well known, such a monitor photosensor typically being used to regulate the electrical drive in such a way as to stabilise the light output of the laser.
For some applications the photosensor is required to have a fast response, and this usually entails a construction of photosensor with a relatively small area of photoresponsive surface, which may typically, have a diameter of less than 50 .mu.m and may be located between 2 and 4 mm from the rear facet of the laser chip. To concentrate sufficient optical power from the laser's rear facet within an area of this size at this distance requires some form of imaging optics, either a lens, a mirror, or both.
Sphere lenses are frequently used with SLEDs and ELEDs and can have a diameter as small as 0.3 mm. Lasers have active regions of around 50 nm and cannot couple successfully with such sphere lenses. The image of the rear facet of the laser on the lens would lie a long way below the principal axis of the lens resulting in an acute transmission angle through the lens and a consequently large image size. One way of using a focusing reflector to direct light from an injection laser on to a photodiode is described in EP-A-0294167. The reflector in that instance is specifically arranged to be deformable so as to allow optimisation of the optical coupling between source and detector. Adjustment of this sort is performed on an individual basis whilst actively monitoring the optical coupling achieved, and thus is both time consuming and costly. Furthermore, the use of such techniques may introduce long-term stability problems. U.S. Pat. No. 4,293,826 (Xerox) provides a laser detector arrangement wherein the optical detector is integral with the substrate and aligned at oblique angles relative to the path of light from one of the light emitting facets of the laser. This arrangement does not focus the light and accordingly fast response photo-diodes cannot be used since such photo-diodes have small active areas. Feedback control is thus not possible for high bit-rates.
The present invention seeks to provide an improved coupling arrangement for the back facet emissions of an injection laser.